1. Technical Field
The present invention relates generally to radio transceivers using phase-locked loop circuits (PLLs). It also relates generally to superheterodyne radio transceivers. In particular the present invention relates to superheterodyne transceivers capable of executing an alignment process without using an external feedback loop.
2. Discussion of Related Art
The growing spread of cellular phones in recent years shows the demand for low cost transceivers. As for the phones, a wireless transmission system is planned to replace connection wires in other technical fields.
One conventional system to replace connection wires is the Bluetooth™ system. It is a low power, low range, and low cost radio communication system that is planned to replace wires or cables between computers, components, e.g. monitors and the like. The Bluetooth™ system operates at a frequency of 2.4 GHz. The frequency band is called ISM (Industrial Scientific and Medicine) and is destined for unlicensed low power radio operation up to a maximum irradiated power up to 100 mW, or 20 dbm. With an irradiated power up to 100 mW a spatial transmission range of a few centimeters up to a few hundred meters can be attained. Due to limitations in different countries, 23 to 79 channels having a bandwidth of 1 MHz are allocated for the unlicensed use. The Bluetooth™ system uses a frequency hopping algorithm to reduce interference caused by other users of the ISM band. The frequency hopping is executed at a rate of 1600 hops per second.
With such a short distance radio transmission all plug-in connection problems associated with e.g. laptop computers having periphery devices such as printers, storage means, or a network are easily solved, thereby obviating the need for a wire connection.
In most transceivers having variable frequency a phase locked loop (PLL) circuit is used to generate the channel frequency. The use of a PLL is a well known and cheap way to build a variable frequency generator having almost the frequency stability of a quartz oscillator.
A standard PLL comprises a voltage controlled oscillator (VCO), a frequency divider, a reference oscillator and a feed back loop. In the standard operation, the output oscillation from the VCO is divided by a frequency divider and the phases are compared with the phases of a reference oscillator in a phase detector. Any deviation of the phases of the divided VCO frequency and the reference oscillation is fed back to the VCO. With this feedback loop the output frequency of the PLL is locked to a multiple of the reference oscillator frequency. Frequency deviations of the reference oscillator due to manufacturing tolerances are multiplied by the division factor of the frequency divider. Therefore it is important to align the reference oscillator.
For low cost radio transceivers the frequency of the reference oscillator is usually adjusted during manufacture using a piece of measurement equipment such as a frequency counter. The frequency is measured while at the same time an internal parameter of the transceiver is varied until the measured frequency reaches the required value. The accuracy of the final frequency is therefore related to the time necessary for the adjustment process and the cost incurred by the final measurement equipment used. This method is used for Bluetooth™ modules during production.
Frequency shift keying (FSK) transceivers e.g. frequency modulation (FM) transceivers must have their modulation index of the transmitting path of the transceiver aligned. It is the usual routine to align the modulation index of an FSK transceiver such that the frequency deviation fulfils a given specification. For example, the specification requires that the maximum and minimum frequency deviations are not allowed to exceed predetermined limits. To achieve this accuracy the transmitter modulation index must be aligned by adjusting the transmitter of the device under test (DUT). The time taken for this alignment and the cost of the equipment required must be related to the accuracy of the final result. The alignment which is eventually adopted uses a spectrum analyzer to measure the output spectrum of the generated signal from the DUT transmitter. This result is employed to calculate the correct setting for the transmitter. The final setting is fed back to the DUT and a further measurement is made to confirm the setting.
The main drawback of the conventional alignment processes resides in the expensive test equipment and a time consuming alignment process. The alignment processes require complex algorithms and external feedback loops. Therefore, a transceiver module that can be sold to customers for integration into e.g. personal computers, has to provide additional external outputs and inputs. A conventional radio frequency transceiver module in addition needs a base band memory capacity.
From the WO 00/18072 there are already known receivers and transmitters for receiving or transmitting a radio packet at a predetermined frequency. A transceiver according to this document permits a power saving operation. It is not capable though of executing fast and simple alignment processes for the operation parameters, such as the reference oscillator frequency or the transmit modulation index.